1. Field of the Invention
This invention relates generally to electronic filters, and more particularly to techniques which may be applied to various conventional filter circuits, such as lumped element filters and cavity filters, to achieve a narrower bandwidth and a higher signal-to-noise ratio and Q transfer function than would normally be attainable using conventional filtering techniques.
2. Description of the Prior Art
An electronic filter is a combination of lumped or distributed circuit elements arranged to have frequency characteristics such that some frequencies are passed while others are blocked. Filters provide an effective means for the reduction and suppression of electromagnetic interference as the spectral content of the signal passed is controlled. The application of filtering techniques requires careful consideration of an extensive list of factors including insertion loss, impedance, power handling capability, signal distortion, tunability, cost, weight, size and the rejection of undesired signals. Often, filters are used as stopgap measures, but if suppression techniques are used early in the design process, the complexity and cost of interference reduction can be minimized.
The types of filters are classified according to the band of frequencies to be transmitted or attenuated. The basic types include low-pass, high-pass, band pass and bandstop (reject). Filters can include lumped, distributed or dissipative elements; the type used is primarily a function of the desired frequency.
Generally, it is best to filter signals at the source of interference, suppress all spurious signals, design filters such that they are essentially independent of environmental variables such as temperature and humidity, and ensure that all filter elements interface properly with other electromagnetically compatible (EMC) elements (which typically involves properly mounting the filter in a shielded enclosure). Filters using lumped and distributive elements are generally reflective, in that the various component combinations are designed for high series impedance and low shunt impedance in the stop band while providing low series impedance and high shunt impedance in the pass band.
In most cases, a spectrum of the input signal seen by a conventional filter is dominated by a desired signal, which is surrounded by undesirable sideband noise and extraneous interference. Under such circumstances, a center frequency of the filter is adjusted to approximate the frequency of the desired signal, and the bandwidth is narrowed to a width that just passes the desired signal and rejects unwanted sideband energy. In many instances, however, the components included in conventional filters cannot be adjusted to such a fine degree as to remove all unwanted sideband contributions. In addition, such adjustments to the components may include extensive and disruptive changes in components, which greatly impact the size and weight of the resulting circuit.
It is an object of the present invention to provide a practical apparatus and method, which can modify the bandwidth and increase the Q transfer function of conventional filters without significantly impacting the size, weight and design of the resulting circuit.
It is another object of the present invention to provide an apparatus and method which can readily be applied to conventional filters to reject noise, interference and spurious signals outside the bandwidth of a desired input signal that exhibits static or dynamic frequencys.
It is yet another object of the present invention to provide an apparatus and method to reject noise, interference and spurious signals in conventional filters outside the bandwidth of a desired input signal, which can readily be reconfigured to achieve variations in noise and dynamic range capability.
It is a further object of the present invention to provide an apparatus and method for rejecting noise, interference and spurious signals in conventional filters outside the bandwidth of a desired input signal having a gain that is selectable according to a chosen bandwidth.
In accordance with one form of the present invention, an Adaptively Peaked Super-Regenerative Filter (APSRF) is provided, which includes an Interference Suppression System (ISS), a summing block, a phase splitter, one or more variable attenuators, three couplers, an amplifier and a conventional filter. The APSRF provides for significant narrowing of the bandwidth and an increase in the signal-to-noise ratio (SNR) and Q transfer function of the filter.
An output signal taken from the filter is sampled by one of the couplers and used as a reference sample signal to the ISS. The ISS includes an adaptive loop, which adjusts the reference sample signal in phase and amplitude to generate a replica signal. The replica signal is designed to cancel an input sample signal coupled from the input signal and applied to the summing block. An output of the summing block provides an error sample signal back to the ISS, which enables the ISS to adaptively optimize its nulling function by adjusting the amplitude and phase of the replica signal.
A copy of the replica signal having a 180-degree phase shift is provided by the phase splitter. This copy is in phase with the input signal, and enables a positive or regenerative feedback of the input signal. The copy of the replica signal is also passed through the variable attenuator, and by altering the magnitude of the regenerative feedback via the variable attenuator, the degree of bandwidth reduction can be adjusted over a broad range. As the bandwidth of the ASPRF 10 is increased or decreased, the regenerative gain of the overall circuit varies. These variations can be reduced by the addition of a second variable attenuator coupled to the output of the filter.
These and other objects, features and advantages of the present invention, will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in conjunction with the accompanying drawings.